How Are Anodized Aluminum Films Colored and Sealed?

Coloring Anodic Oxide Films

A freshly anodized film can be colored promptly to improve appearance and corrosion resistance. Coloring followed by sealing produces many colors and improves corrosion resistance and wear resistance.

Chemical coloring of anodized aluminum relies on adsorption by the porous film. Typical pores are 0.01 to 0.03 μm in diameter, while dye molecules separated in water are 0.0015 to 0.0030 μm. Dye is adsorbed on pore surfaces, diffuses and accumulates within the pores, and bonds to alumina through ionic and hydrogen bonds. Sealing fixes the dye in the pores.

Inorganic-Pigment Coloring

Inorganic-pigment coloring operates mainly by physical adsorption: pigment molecules fill and adsorb on micropore surfaces. Colors are not vivid and adhesion to the substrate is poor, but lightfastness is good. Two solutions are used. After thorough rinsing, the anodized metal is immersed in solution 1 for 10 to 15 minutes, rinsed, and then immersed in solution 2 for 10 to 15 minutes until the salts have reacted within the film to produce enough pigment for the required shade.

Organic-Dye Coloring

Organic-dye coloring involves physical adsorption and chemical reactions. Alumina may form covalent bonds with sulfonic groups, hydrogen bonds with phenolic groups, or complexes with dye molecules. Organic dyes provide vivid colors and a broad color range but have poor lightfastness. Distilled or deionized water is preferred because calcium and magnesium ions in tap water form complexes with dye molecules and can ruin the bath. The dye is dissolved in warm water, heated nearly to boiling, and filtered if suspended matter or sediment remains. Bath pH must be monitored and adjusted. Suitable tank materials include ceramic, stainless steel, and polypropylene.

Electrolytic Coloring

Anodized aluminum is electrolyzed in a metal-salt solution. Electrochemical reactions reduce heavy-metal ions that enter the micropores to metal atoms, which deposit in the porous layer at the pore bottoms. Differences in ion type, deposited metal, particle size, and distribution produce selective absorption and reflection at different wavelengths and therefore different colors.

Integral Coloring

Aluminum and aluminum alloys can be anodized in an organic-containing electrolyte so that coloring occurs simultaneously. Fine particles dispersed along the pore walls scatter incident light and create different colors. The particles originate from the substrate or from decomposition products of the electrolyte’s organic compounds. Color depth is directly related to film thickness. High anodic current density and voltage make the process energy intensive.

Sealing Anodic Oxide Films

Anodic films have a porous, highly active surface. Contaminants and corrosive substances can enter the pores, while pigments can escape, reducing surface performance. Whether colored or not, anodized aluminum must therefore be sealed promptly. Sealing fixes dye in the micropores, prevents bleeding, and improves wear resistance, lightfastness, corrosion resistance, and electrical insulation.

Hot-Water Sealing

Hot-water sealing relies on hydration of amorphous Al₂O₃:

[TO VERIFY: hydration equation missing from the source]

In the source, n is 1 or 3. When Al₂O₃ hydrates to aluminum oxide monohydrate, Al₂O₃·H₂O, its volume increases by about 33%. Formation of aluminum oxide trihydrate, Al₂O₃·3H₂O, nearly doubles the volume. Hydration of Al₂O₃ on the film surface and pore walls therefore expands the material and closes the pores.

Hot-water sealing uses water at 90 to 100°C, pH 6 to 7.5, for 15 to 30 minutes. Distilled or deionized water is required because scale from tap water can be adsorbed in the pores and reduce transparency and color quality. Neutral distilled water can produce a cloudy appearance and reduce gloss; slightly acidic distilled water produces better sealing. Steam sealing works by the same principle and is more effective but more expensive.

Dichromate Sealing

Aluminum products are placed in a strongly oxidizing potassium dichromate solution at elevated temperature so that the oxide film reacts with the dichromate. When an anodized part enters the solution, alumina in the film and pore walls reacts with aqueous potassium dichromate. [TO VERIFY: reaction equation missing from the source.]

The resulting basic aluminum chromate and basic aluminum dichromate precipitates, together with aluminum oxide monohydrate and trihydrate produced by hot-water hydration, close the micropores. The bath contains 50 to 70 g/L potassium dichromate and operates at 90 to 95°C for 15 to 25 minutes at pH 6 to 7. The treated film is yellow and has good corrosion resistance. The method suits protective anodizing but not decorative colored films.

Hydrolysis Sealing

When very dilute nickel- or cobalt-salt solutions are adsorbed by the film, hydrolysis occurs. [TO VERIFY: hydrolysis equations missing from the source.] The resulting nickel or cobalt hydroxide precipitates in and seals the micropores. Small amounts are nearly colorless and transparent, making the method particularly suitable for colored protective-decorative films. The hydroxides also complex with organic dyes and improve lightfastness.

Impregnation Sealing

Anodic oxide films can also be sealed with organic materials such as transparent varnish, molten paraffin wax, various resins, and drying oils.